Georges emmanuel zelger



Patented May 10, 1927.

UNITED STATES PATENT oFricE,

GEORGES EMMANUEL ZELGER, OF VINCENNES, FRANCE, ASSIGNOB TO PATHE CINEMA,

ANGIENS ETABLISSEMENTS PATHE FRERES, OF PARIS, FRANCE.

PROCESS FOR OBTAINING DIRECT POSITIVES BY REVERSAL' No Drawing.

The known method for obtaining direct positives or counter-types by reversal, consists in a thorough development of a photographic plate. and in then treating 1t in a 5 bath which dissolves the silver of the image, and lastly in reducing by a developer the bromide of silver which remains unchanged. The ohtainment of the first and thorough development requires a very strong developer. and it is thus necessary to operate as a rule in a very alkaline medium. But the baths for dissolving the silver. based upon potassium permanganate and 'sulphurlc or chromic acids, etc. arealways strongly acid. Inversely, the second development,

effected in a developer. is alkaline.

The applicant has found that this alternation of media, which are alternately strongly acid and basic, to which the gelatine is subiected, tends to alter it considerably, chiefly in warm weather; and 1t 19 especially the operation in the last developing bath which is to be feared inthls respect. The present invention consists in substituting for the second development a chemical reduction. partial or complete, of the silver bromide, by means of baths which have not the nature of developers but which are formed of substances reducing the sllver salt in an acid medium. In this manner an alternation between acid and basic substances is obviated, and after the treatment in the first developer. the gelatine of the print is treated only in an acid medium.

, This avoids the serious drawback above men- ,"tioned.

" 1 The alkaline hydrosulphites, with the addition of a weak. acid, the acidulated Application filed April 14, 1923, Serial No. 632,135, and in Franee'Februar -y 16, 1923- hypoph0sphites,-- stannous chloride, etc. are adapted for this last operation.

By way of example, use can be made of the following li'ath: water. 1 litre, sodium bisulphite liquid, cubic centimeters, sodium hydrosulphite, -10 grammes.

Having" now described my invention. what I claim as 'new and desire to secure by Letters Patent is:

1. A photographic process for obtaining direct positives consisting in effecting a first development. then dissolving in an acid bath the silver of the image thus obtained, and

then effecting the final reduction of the remaining silver salt in a non basic bath containing substances capable of reducing said silver salt in an acid bath even without any exposure to light.

2. A photographic process for obtaining direct positives consisting in efi'ecting a first development, then dissolving in an acid bath the silver of the image thus obtained and then efiecting the final reduction of the remaining silver salt in an acid bath containing substances capable of reducing said silver salt in an acid bath even without any exposure to light.

3. A photographic process for obtaining direct positives consisting in effecting a first development, then dissolving in an acid bath .5

the silver of the image maining silver'salt'in a nonba sic bath con taming 1n proportion: Water lilrtre, sodiu thus obtained and? then effecting the final reduction of the rebisulphite liquid 50 cubic centimeters, sodium 7 hydrosulphite 10 grammes. y

In testimonyi whereof I have signed m name to this specification. T

GEORGES-EMANUEL Z ELGER. 

